Group Additivity Calculations

Oxidation of unsaturated and aromatic hydrocarbons in atmospheric and combustion processes results in formation of linear and cyclic unsaturated, oxygenated-hydrocarbon intermediates. The thermochemical parameters, A and Cpj T) for these 

In the previous chapter. Density Functional calculations have been used to determine the thermodynamic properties of specific species. A second application is considered in this chapter which allows developing new group for group additivity application. A consistent set of oxygenated peroxy-hydrocarbon and acetylene-alcohols groups are calculated. 

The thermochemistry of a series of unsaturated oxygenated and non-oxygenated species and radicals is first determined. The molecules are selected because they are small and contain the unknown groups which are our target groups in this chapter. 

This thermochemistry is then used to calculate thirty-one new groups which are needed to evaluate large molecules system (larger unsaturated oxygenated or multi-oxygenated intermediates) where possible calculation techniques are less accurate. 

The GA method serves as a fast and accurate estimation technique for many scientists and engineers whose work involves thermodynamic characterization of elementary and overall reaction processes. One convenient way to perform Group additivity calculations is by using the THERM code for hydrocarbons distributed by Bozzelli [81, 82]. 

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Another technique employs a database search. The calculation starts with a molecular structure and searches a database of known spectra to find those with the most similar molecular structure. The known spectra are then used to derive parameters for inclusion in a group additivity calculation. This can be a fairly sophisticated technique incorporating weight factors to account for how closely the known molecule conforms to typical values for the component functional groups. The use of a large database of compounds can make this a very accurate technique. It also ensures that liquid, rather than gas-phase, spectra are being predicted. 

Direct measurements of the equilibria (47, -47) brought into question previous estimates of the thermochemistry of the R + O2 system which had been based on group additivity (Chapter 1) calculations. These calculations were subject to considerable uncertainty as prior to the mid 80 s there was little direct experimental evidence on AfH R) or ArHR+oi-Group additivity calculations predicted an almost constant bond energy (or with increasing complexity of the alkyl radical. In contrast... 

An example of group additivity calculation of A// 298 for CH3CH=CH2 is as follow ... 

Enthalpy, entropy and heat capacities are determined by using DFT and ab initio methods (G3 and G3MP2B3). Our calculated values were compared to the vinyl + O2 calculation data of Mebel et al., to the phenyl + O2 calculation data of Hadad et al. and to recent data for phenyl + O2 by Tokmakov et al. Group additivity calculations were performed as well to test our group additivity values. 

In several instances, group additivity calculations have highlighted experimental measurements that are almost certainly in error. The technique thus not only allows prediction of unmeasured thermochemical properties, but provides a simple standard by which to judge the probable accuracy of published data. 

Error is difference between group additivity calculation and values tabulated by SWS (Reference 22). 

The primary problem with explicit solvent calculations is the significant amount of computer resources necessary. This may also require a significant amount of work for the researcher. One solution to this problem is to model the molecule of interest with quantum mechanics and the solvent with molecular mechanics as described in the previous chapter. Other ways to make the computational resource requirements tractable are to derive an analytic equation for the property of interest, use a group additivity method, or model the solvent as a continuum. 

The simplest empirical calculations use a group additivity method. These calculations can be performed very quickly on small desktop computers. They are most accurate for a small organic molecule with common functional groups. The prediction is only as good as the aspects of molecular structure being par-... 

The solubility parameter is not calculated directly. It is calculated as the square root of the cohesive energy density. There are a number of group additivity techniques for computing cohesive energy. None of these techniques is best for all polymers. 

Computed optical properties tend not to be extremely accurate for polymers. The optical absorption spectra (UV/VIS) must be computed from semiempiri-cal or ah initio calculations. Vibrational spectra (IR) can be computed with some molecular mechanics or orbital-based methods. The refractive index is most often calculated from a group additivity technique, with a correction for density. 

Neglecting end group effects, calculate for each of these polymers from the end group data. Are the trends in molecular weight qualitatively what would be expected in terms of the role of the additive in the reaction mixture Explain briefly. 

Two standard estimation methods for heat of reaction and CART are Chetah 7.2 and NASA CET 89. Chetah Version 7.2 is a computer program capable of predicting both thermochemical properties and certain reactive chemical hazards of pure chemicals, mixtures or reactions. Available from ASTM, Chetah 7.2 uses Benson s method of group additivity to estimate ideal gas heat of formation and heat of decomposition. NASA CET 89 is a computer program that calculates the adiabatic decomposition temperature (maximum attainable temperature in a chemical system) and the equilibrium decomposition products formed at that temperature. It is capable of calculating CART values for any combination of materials, including reactants, products, solvents, etc. Melhem and Shanley (1997) describe the use of CART values in thermal hazard analysis. 

A typical maleimide resin is synthesized by the Michael addition of MDA and BMI (Fig. 4). If the stoichiometrically equal amounts of MDA and BMI are added into the reaction solvent under controlled temperature, linear, high molecular weight polyaminoimide (PAI) results. To obtain crosslinkable oligomer (pre-polymer) with maleimide end groups, a calculated 1.1-1.8 times an excess... 

Where no data exist, one wishes to be able to estimate thermochemical quantities. A simple and convenient method to do that is through the use of the method of group additivity developed by Benson and coworkers15,21 22. The earlier group values are revised here, and new group values calculated to allow extension of the method to sulfites and sulfates. In addition, a method based on the constancy of S—O bond dissociation energies is applied. 

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